It is really amazing just how many times this comes up on the forum. You've spent a small fortune on your CPU, RAM, Motherboard Graphics Card, PC Case etc go and connect it all up and..... it doesn't work!The first thing you do is check all the cables are connected, still nothing.You rip everything out and replace them all one by one.... .still nothing.Must be a problem with the motherboard, CPU, RAM, Graphics Card.... right?Well, being brutal, probably not. Have you stopped and thought of everything? Really??What about your PSU? That's right the box that sits in the PC Case and supplies power to your system.The majority of problems when building a PC, come down to the choice of Power Supply. If you consider that the CPU and RAM are the brains of your computer and that the motherboard is the skeleton then the PSU is the heart of it, pumping life giving energy to all of the components. In today's computers you need a good, strong steady flow of power or things just won't work properly.It is no good spending all of your budget on your CPU, RAM, Motherboard Graphics Card and other parts without taking into consideration the power you are going to need to keep things running. Most motherboards require a minimum of 500 Watts and that's just for the basics. Having spent your money on other things it really isn't advisable to spend the least amount possible on your Power Supply.A 500 Watt or less power supply might well be adequate for your needs but this should still be a good quality one not the least expensive. It should also be the minimum that you are looking at, especially if you are considering adding extra hard-drives, High End Graphics Card, DVD/CD Burners etc. You should actually be looking at 650 Watts as a minimum for most of today's PC's.Things to look for when considering your Power Supply:Does it have a Single rail or multiple rails? You are better off with a single rail that supplies enough Amps rather than two or more rails supplying the same power across them equally.Does it have all of the necessary connectors? Most modern motherboards need a 24 PIN ATX connector and also a 4 or 8 pin ATX 12V connector. If you are using a high end CPU and Graphics card you really need to connect an 8 PIN ATX 12V connector to supply enough power for your CPU and Graphics Card's PCI-E slot, if your motherboard has this connection type. Also you may need extra PCI-e power connectors for your Graphics Card(s) so check that your chosen PSU has these. Although you can get away with using adaptors this adds another possible area for problems to occur.How does it compare with other Power Supplies? Do your research. Often a quick search on the internet will provide you with all the information you need, as to how good a certain make of PSU is.Does it come with a good warranty? Look for a PSU with at least a 3 year warranty, if you do encounter problems with it then at least you know you're covered.Remember that, over time and with normal usage, the power supply will decrease in efficiency. Just because it worked in your old system doesn't mean it is good enough for your new one.Where possible, buy a PSU with a higher rating than you think you actually need. This will allow for future expansion and will also give some leeway for the natural decrease in efficency over time.